This invention relates generally to measurement systems and, more specifically, to a method and gauge for measuring sharpness.
Form mats with grids of sharp pins are used for forming acoustical surfaces in many applications, including aircraft engine cowls. These pin mats perforate the material being fabricated during lay-up and curing, forming an acoustically dampening surface. The pins on the pin mats are typically quite small and closely spaced. The mats often have more than 100 pins per square inch. If the pins are dull, they do not perforate the material being formed, resulting in a defective acoustical surface. While a certain percentage of perforation failures are acceptable, a large number of pin failures results in the formed material being rejected.
Pin mats are typically manufactured by injection molding (not shown), but may be manufactured by other methods. In injection molding, the pin mat mold has vents which allow air to exit as the plastic is injected into the mold. If a vent is clogged or if an injection port is restricted during the filling stage of the injection process, which takes approximately 3 seconds, pins in part of the mat may not be properly formed. Each pin is typically molded with a conical tip. The nature of the injected polypropylene is such that the tip of each pin, as plastic is injected into the conical pin voids in the pin mat mold, forms a cone up to a certain distance from the tip, and then becomes spherical. Variations in the molding process parameters of the pins cause the conical portion of the pin to be filled to a varying degree. The molded polypropylene then forms a spherical radius at the tip of each molded pin, with varying radii. The smaller the spherical radius at the tip of a pin, the sharper the pin. The larger the pin tip radius, the duller the pin.
A dull pin will buckle or fail when the pin mat holding the pin is pressed against or into composite materials being molded with an acoustical surface. Typically, the pin mats are used only once. The composite materials being formed are cured with the pin mat in place. Removal of the pin mat typically results in distortion of the pins, preventing reuse of the pin mat. Multiple mats are often used in the forming of composite components, such as the inner walls for the engine ducts on commercial aircraft.
Pin failures are directly related to pin tip sharpness. A dull pin will not perforate the material being formed, while a sharp pin will. Historically, pin tip sharpness has been measured indirectly. The pin mats are used in a forming process, and are rejected when the formed materials are improperly perforated. Composite materials used in forming special shapes, especially for aircraft parts, are expensive and involve substantial lay-up time. The rejection of formed composite pieces due to inadequate pin tip sharpness can be quite expensive.
Other prior methods of pin tip sharpness measurement include optical inspection and the use of a coordinate measuring machine. Optical inspection, pin by pin, such as by using a magnifier, is time consuming, and involves observer subjectivity, or at minimum, the potential for observer error. Direct coordinate measuring of pins, such as by a laser coordinate measuring machine, is very accurate, measuring surface differences down to 0.001 inch, but is very slow. It takes a considerable amount of time to coordinate measure even a single pin, let alone a large sample of pins to permit the grading of a pin mat used for forming composites.
Therefore, a need exists for quickly and easily measuring the sharpness of objects, especially pin tip sharpness.
A method and a gauge for measuring the sharpness of an object is provided. The method includes receiving the tip of the object to be measured in the end of a tube. The object deflects a plunger in the tube a finite distance proportional to the sharpness of the object inserted in the tube. Measuring the sharpness of the tip of the object is accomplished by measuring the deflection distance of the plunger, and deriving the sharpness from the deflection distance. The sharpness of the object is then displayed. The measuring method is used to quickly measure the sharpness of a number of objects in sequence, such as where the objects being measured are pins arranged in a matrix pattern.
According to an aspect of the invention, a gauge for measuring the sharpness of the tip of an object includes a tube having an opening at an end of the tube and the opening is defined to receive the tip of an object. A plunger is disposed inside the tube, and the plunger is deflected a finite distance inside the tube when the tip of the object is inserted into the tube. The gauge has a means for displaying the sharpness of the object.
According to another aspect of the invention, the present invention also provides a method for sampling the sharpness of pins on a pin mat used for forming composite or molded materials. The method and the gauge of the present invention may be used to sample a predetermined set of pins in a forming mat, and displaying the sharpness measurements directly, or compiling the sharpness measurements into a sharpness map used for grading the pin mat. The invention provides a rapid method of measuring the sharpness of objects, especially multiple objects, and rapidly displaying and mapping the results of such measurements. The present invention allows rapid assessment of pin tip sharpness prior to the use of pin mats for forming composite or molded materials. This can substantially reduce rejection of formed materials due to improper pin tip sharpness in the pin mats.